Enzymes assays are playing an important role in the detection and isolation of leads for anticancer drugs. One of these enzymes is IMPDH, of which abnormally high concentrations were found in ovarian tumor and leukemic cells. Mycocphenolic acid is the most potent inhibitor for IMPDH and used as a standard for detection of other inhibitors. Extracts from a variety of sponges give positive results in IMPDH enzyme assay. To provide a comprehensive picture of structure-activity relationship, the structure of the isolated inhibitors was determined. All IMDPH inhibitors in this report cen be generically classified as unsaturated acids. One of these is a new acetylenic acid (pellynic acid) which was isolated along with several related alcohols which were inactive. Tandem mass spectrometry was used in the structure proof.